This invention relates to a variable gain amplifier circuit including an operational amplifier circuit.
This invention more specifically pertains to a tone quality regulating device for use in audio equipment such as a stereophonic amplifier.
In a conventional stereophonic amplifier, a tone quality regulation is carried out by directly processing an audio signal by the use of variable resistors or by providing discrete circuits each of which is activated by a corresponding manual switch. However, the former case is disadvantageous in that wiring to the tone control may pick up noise while the latter case is disadvantageous in that the number of circuit elements is high for the function to be performed.
A variable gain amplifier circuit employing a digital control system has been known in the art in which the gain is varied by a digital code signal. Such a variable gain amplifier circuit is as shown in FIG. 1. In FIG. 1 reference character OP.sub.1 designates an operational amplifier circuit whose non-inverting input terminal is grounded. A feedback resistor Rf.sub.1 is connected between the inverting input terminal and the output terminal of the amplifier circuit OP.sub.1. Connected to the input terminal of the amplifier circuit is a parallel circuit of resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4 which are provided in correspondence to the number of bits in the digital code signal, for instance as shown here, four bits. First ends or contacts of analog switches SW.sub.1, SW.sub.2, SW.sub.3 and SW.sub.4 are connected to the resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4, respectively, and second contacts of the switches are connected to the inverting input terminal of the operational amplifier OP.sub.1. In the circuit thus constructed, the analog switches SW.sub.1 through SW.sub.4 are subjected to on-off control according to the contents of the four-bit digital code signal so that the amount of feedback to the operational amplifier OP.sub.1 is correspondingly changed thereby controlling the gain.
Such a variable gain amplifier circuit has been employed as a so-called "logarithmic amplifier" circuit in which the output voltage is a logarithmic function of the input voltage. However, such use of the circuit is somewhat disadvantageous in that since that system utilizes a polygonal line approximation system, the gain cannot change in a logarithmic linear manner with respect to variations in the amount of feedback of the operational amplifier OP.sub.1.
Accordingly, an object of this invention is to provide a variable gain amplifier circuit the gain of which can change in a logarithmic linear manner with respect to variations in the amount of feedback of its associated operational amplifier.
An object of this invention is also to provide a tone quality regulating device capable of eliminating the drawbacks mentioned above which are present in a conventional device of this category.
Hereinafter, an embodiment according to this invention will be described with reference to the accompanying drawing.